1. Field of the Invention
This invention relates to a memory card device having an electrically erasable and programmable read only memory (EEPROM) as a semiconductor memory contained in a card-form case, and more particularly to a memory card device suitable for use in an electronic still camera device or the like for converting an optical image of an object photographed into digital image data and storing the data into a semiconductor memory.
2. Description of the Related Art
In recent years, as is well known in the art, an electronic still camera device has been developed which converts an optical image of a photographed object into an electrical image signal by use of a solid image pickup device such as a charge coupled device (CCD), then converts the electrical signal into digital image data and storing the image data into a semiconductor memory. This type of electronic still camera device is so designed that a memory card having a semiconductor memory contained in a card-form case can be removably attached to a camera body and can be treated in the same manner as a film cartridge in a normal camera.
The memory card used in the electronic still camera device is standardized, and it must have a sufficiently large memory capacity for storing a plurality of image data items. The semiconductor memory contained in the card can be either static random access memory (SRAM), mask ROM or EEPROM in which data can be electrically programmed and erased, for example. A memory card using an SRAM has already been commercialized.
The memory card using an SRAM has advantages such as that data structures of any format can be used and that data write-in and readout speeds are high, but since SRAMs require a back-up battery to hold written data in the memory card, the memory capacity is reduced by an amount corresponding to the space occupied by the battery. Further since the cost of the SRAM itself is high, it is economically disadvantageous.
At present, in order to solve the problems caused by use of the SRAM, the use of an EEPROM as the semiconductor memory used in the memory card has received much attention. Previously, the EEPROM received much attention as a recording medium which can be used instead of the magnetic disk. The EEPROM has advantages over the SRAM in that it is not necessary to use a back-up battery for holding data and the cost of the chip itself is low. For these reasons, various studies have been conducted on the EEPROM so that it can also be used for the memory card.
FIG. 1 shows the advantages and disadvantages of the memory card (SRAM card) using the SRAM as compared with those of the memory card (EEPROM card) using the EEPROM. As described before, in items 1 and 2 for comparing the back-up battery and cost, the SRAM card necessitates a back-up battery and the cost thereof is high, and the EEPROM card does not necessitate a back-up battery and the cost thereof is low.
In items 3 and 4 for comparing the write-in speed and readout speed, a random access mode is commonly set for the SRAM and EEPROM, and in which data is written and read out in the unit of bytes or bits freely specified by an address also shown for comparison is a page mode which is inherent to the EEPROM and in which data is simultaneously written and read out in the unit of page by specifying a page formed of a plurality of successive bytes (several hundreds of bytes).
In the random access mode, the write-in speed and readout speed of the SRAM are both high and the write-in speed and readout speed of the EEPROM are both low. In the page mode of the EEPROM, since a large amount of data of one page can be simultaneously written in or read out, the data write-in speed and readout speed are enhanced in comparison with the case of the random access mode.
Further, the erase mode in item 5 is inherent to the EEPROM and is not provided in the SRAM. That is, when data is newly written into an area in which data is already written or data is re-written in the EEPROM, the new data cannot be written if the previously written data is not erased. Therefore, at the time of re-writing data in the EEPROM, the erase mode is effected.
In the erase mode, the simultaneous erasing operation for simultaneously erasing all the data stored in the EEPROM or the block erasing operation for erasing data for each block unit by specifying a block constructed by a plurality of successive pages is effected.
The write-in verify mode in item 6 is also inherent to the EEPROM and is not provided in the SRAM. That is, in the general data write-in operation, data cannot be completely written by one write-in operation. For this reason, each time data is written into the EEPROM, the operation of reading out the written data from the EEPROM and then checking whether the data is correctly written is necessary.
More specifically, the write-in verifying operation is effected by storing data to be written into the EEPROM in a buffer memory, transferring the data from the buffer memory and writing the data into the EEPROM, reading out the written data from the EEPROM and comparing the readout data with the content of the buffer memory to determine whether the compared contents coincide with each other. When the result of verification indicates non-coincidence (error), the operation of writing the contents of the buffer memory into the EEPROM again, reading out the written data and comparing the readout data with the contents of the buffer memory to determine whether the compared contents coincide with each other is repeatedly effected.
As is clearly understood from the above explanation, the EEPROM has its own advantages, which the SRAM does not have, in that the back-up battery is not necessary, the cost of the chip itself is low, and data can be written and read out in page units, but has disadvantages that the data write-in speed and readout speed in the random access mode are low and the erase mode and write-in verify mode which the SRAM does not necessitate are required.
When considering use of an EEPROM instead of an SRAM as a semiconductor memory for use in a memory card, it is important to solve the above-discussed problems that the data write-in speed and readout speed are low and the erase mode and write-in verify mode are necessary and to improve the detail portions of the EEPROM card so that the EEPROM card can be used in substantially the same manner as in a memory card using the SRAM.
In this case, there is a serious problem in that defects tend to occur in the memory cells of the EEPROM after a certain number of operations of re-writing data unlike the case of the SRAM and DRAM and a defective storage area in which data cannot be correctly written and stored will form. This is because the EEPROM has been developed for storing program data and intended only to re-write data at the time of version-up of the program data and is not designed to cope with a large number of data re-writing operations.
However, as described above, when the EEPROM is used instead of the SRAM in a memory card for an electronic still camera device, for example, data is frequently re-written into the EEPROM, and as a result, the generation rate of write-in defects will increase without fail.
Conventionally, the occurence of a write-in defect is defined as when data is not correctly written even after the write-in verify operation is repeatedly effected by a preset number of times. In the prior art, even when the write-in defect occurs only in part of the EEPROM, the entire memory card containing the EEPROM is treated as a defective card, and therefore, the manufacturing yield thereof is extremely low and it is economically disadvantageous. Overcoming the problem of multiple write-in defects is therefore critical to the success of a memory card device using an EEPROM as a semiconductor memory.